An aerosol is generally a colloid in which a solid or liquid as a dispersoid is dispersed in a gas as a dispersion medium, and examples of such an aerosol include car exhaust.
As an apparatus for classifying fine particles contained in an aerosol as a measuring object, an apparatus designed to electrically charge an aerosol and classify charged fine particles contained in the aerosol by utilizing the difference in their mobility in an electric field is known. As an apparatus for measuring fine particles, an electric mobility analyzer designed to measure the electric mobility of classified fine particles is known. There is a certain relationship between the electric mobility and the particle size of a fine particle, and therefore by specifying an electric mobility, it is possible to specify a particle size.
Electric mobility analyzers are divided into two types: differential mobility analyzers (DMA) (see Patent document 1) capable of separating fine particles having a certain electric mobility and electric mobilities in its immediate vicinity, and integral mobility analyzers capable of separating fine particles having electric mobilities less than a certain electric mobility.
The particle size distribution of fine particles contained in car exhaust generally has two characteristic modes, a nuclei mode and an accumulation mode. The particle size distribution of nuclei mode particles has a peak of about 10 nm, and the particle size distribution of accumulation mode particles has a peak of about 70 nm and ranges from about 30 to 100 nm. The nuclei mode particles and accumulation mode particles are different in generation mechanism and composition, and therefore need to be separated from each other to measure both of the total number of nuclei mode particles and the total number of accumulation mode particles in real time.
In order to measure accumulation mode particles, a measuring apparatus needs to have such passage characteristics that it passes fine particles having particle sizes in the range of, for example, 30 to 100 nm at a constant passage rate but does not pass fine particles having particle sizes smaller than 30 nm and fine particles having particle sizes larger than 100 nm.
Patent document 1: Japanese Patent No. 3459359